2 Corinthians 9:7 So let each one give as he purposes in his heart, not grudgingly or of necessity; for God loves a cheerful giver.
We are called to be servants of God, but if we are not going to do so willingly then we are saying that we are slaves.
I think most of us have grown up knowing what slavery is nowadays and we would not like to be slaves to anyone; but there are many people who still are, even in this modern world.
God wants us to come forward willingly to work for Him, just as we would willingly give money to people who need it or to the church to help with its running.
God does not want us as slaves. He overcame that for us.